1. Field of the Invention
The present invention relates to a disc rotation control circuit for controlling a rotation means that rotates a disc.
2. Description of the Related Art
In recent years, various optical recording/reproduction apparatuses have been developed which record or reproduce information onto or from a 3.5-inch CD (compact disc), MO (magneto-optical) disc, or the like.
On the other hand, more compact magneto-optical recording/reproduction apparatuses have been developed for use in personal audio equipment (portable, stationary or vehicular equipment). For example, a MD recording/reproduction apparatus is known which records or reproduces information onto or from a recording medium (hereinafter called "MD") in which a 64-mm read-only (reproduction-only) optical disc (ROM disc), readable/writable, i.e., rewritable MO disc, or hybrid disc (partial ROM disc) having a readable/writable (i.e., rewritable) region and a read-only region is incorporated in a cartridge.
In this MD recording/reproduction apparatus, when the MD is a MO disc or a hybrid disc, data is recorded according to a magnetic modulation overwrite recording scheme.
On the other hand, reproduction from the MD is performed in the following manner. In the case of a ROM disc having the same structure as a CD, a reproduction signal is detected by utilizing diffraction of light by a bit array of a target track. In the case of a readable/writable magneto-optical disc, a reproduction signal is detected by detecting a difference in the polarization angle (Kerr rotation angle) of reflection light from a target track. In the case of a hybrid disc, for a read-only region (ROM region), a reproduction signal is detected by utilizing diffraction of light by a bit array of a target track in the same manner as in a ROM disc. For a readable/writable region (magneto-optical region), a reproduction signal is detected by detecting a difference in the polarization angle (Kerr rotation angle) of reflection light from a target track in the same manner as in a magneto-optical disc.
In the development process of personal audio equipment, circuit elements of the above MD recording/reproduction apparatus have been integrated and parts of its mechanisms have been optimized, to make the entire apparatus more compact and lighter. Further, the MD recording/reproduction apparatus is now capable of battery-powered operation because of its low power consumption. Further, in addition to the advantages that the MD has a storage capacity (120 Mbytes) that is approximately equal to that of an existing 3.5-inch MO disc and that different types of MDs can be used interchangeably, the manufacturing cost of the apparatus and the recording medium can be reduced by virtue of mass-production effects as compared to other MO disc drives. In addition, the reliability of the MD recording/reproduction apparatus is sufficiently assured from its past performance when used in personal audio equipment.
By the way, in the above-described conventional MD recording/reproduction apparatus, data that has been subjected to error correction coding, i.e., the ACIRC (advanced cross interleave Reed-Solomon code) coding and modulation suitable for recording, i.e., the EFM (eight-to-fourteen modulation) is recorded onto and reproduced from a disc being subjected to the rotation control of constat linear velocity (hereinafter called "CLV control").
In the optical recording/reproduction apparatus for a CD, the CLV control is performed according to the linear interpolation oversampling scheme while the rotation speed of a spindle motor for rotating a CD is detected. In the optical recording/reproduction apparatus for a MO disc, the CLV control is performed by using an analog LPF (lowpass filter) while the rotation speed of a spindle motor for rotating a MO disc is detected.
Further, the CLV control includes a mode for controlling only the rotation speed (hereinafter called "CLVS control") that is performed at a start of rotation of a spindle motor, for instance, when a disc is inserted, and a mode for controlling both of the rotation speed and the phase (hereinafter called "CLVP control") that is performed after the end of the CLVS control.
Japanese Patent Application No. Hei. 4-130986 (filed May 22, 1992; a corresponding U.S. patent application is now pending) and U.S. Pat. No. 5,054,014 (patented Oct. 1, 1991) relate to the disc rotation control apparatus of this invention, each of which is owned by the present assignee and hereby incorporated by reference.
Also the above MD recording/reproduction apparatus has the CLVS control mode and the CLVP control mode. Further, the MD recording/reproduction apparatus has two modulation schemes: EFM (for a ROM disc) and ADIP (address in pregroove; for a MO disc or a partial ROM disc). That is, the MD recording/reproduction apparatus is required to perform the four kinds of CLV control for the two modulation schemes and the two CLV control schemes.
The conventional MD recording/reproduction apparatus accommodates the two CLV control modes also by different control circuits. Therefore, to perform the four kinds of CLV control of EFM-CLVS, EFM-CLVP, ADIP-CLVS and ADIP-CLVP, the configuration of the control circuits (particularly loop filters (lowpass filters)) becomes complex. Thus, there is a problem that it is difficult to realize a compact apparatus, which is expected of compact personal audio equipment.